1. Field of the Invention
This invention relates to a process for regulating the molecular weight and physical properties of fluoro-olefin polymers, hereinafter called fluoro-polyolefins. This invention is particularly directed to obtaining polyfluorovinyl compounds which are workable thermoplastically in known manner. This invention is also directed to obtaining copolymers of these fluoro-olefins with other compounds which can be copolymerized with such fluoro-olefins. This invention is directed to the fluoro-polyolefins themselves of which are workable thermoplastically.
2. Discussion of Prior Art
Fluoro-polyolefins such as, for instance, polyvinylidene fluoride (PVF.sub.2), polyvinyl fluoride (PVF) or copolymers consisting predominantly of vinylidene fluoride (VF.sub.2) or vinyl fluoride (VF) can be used in many fields because of their excellent weather resistance and light stability as well as their good resistance to chemicals, heat and cold. In this connection the polymers having a high molecular weight are used preferably as organosols for coating metals or for making cast sheets. However, for thermoplastic molding, e.g. calendering, extruding or injection molding, products having a medium-high molecular weight are more suitable.
Depending on the polymerization process used, fluoro-olefin polymers have molecular weights of varying size. The result is that the products obtained, which have a high molecular weight, are neither genuinely soluble in known solvents nor thermoplastically workable below their decomposition temperature. In particular, polymers, which are made with water-soluble, free radical initiators in the aqueous phase, with or without additional emulsifier, in a temperature range between 50.degree. and 150.degree. C, have as a rule a molecular weight that is too high for thermoplastic processing. Generally known methods for the reduction of the molecular weight, e.g. increase in the amount of initiator or the polymerization temperature, are without noticeable effect or give rise to undesirable side effects such as, for instance, an impairment of the thermal stability. Therefore, in order to reduce the molecular weight it is necessary to use chain transfer agents -- which are also called polymerization regulators.
Some compounds have already been proposed as regulators, for instance, trans-dichloroethylene or isopropanol for the polymerization of vinyl fluoride, isopropanol or ethylene oxide for the polymerization of vinylidene fluoride. All of these compounds have the disadvantage of either regulating too little, i.e, not all of the desirable molecular weights can be attained, or partially inhibiting in addition the polymerization, i.e., the polymerization rate is greatly reduced. Also, some of the known regulators can effect the desired reduction of the molecular weight only in a specific polymerization process, e.g., at high pressure, high speed of agitation, etc., without affecting too much both reaction rate and yield. Other known polymerization regulators such as mercaptans, tetrahydrofuran, chlorinated hydrocarbons and others, interfere in particular with the vinylidene fluoride polymerization to such an extent that a polymerization under mild conditions, e.g, at pressures of less than 100 atmospheres, is practically impossible.